The origin of IRS 16: dynamically driven inspiral of a dense star cluster to the Galactic center?

We use direct N-body simulations to study the inspiral and internal evolution of dense star clusters near the Galactic center. These clusters sink toward the center due to dynamical friction with the stellar background, and may go into core collapse before being disrupted by the Galactic tidal field. If a cluster reaches core collapse before disruption, its dense core, which has become rich in massive stars, survives to reach close to the Galactic center. When it eventually dissolves, the cluster deposits a disproportionate number of massive stars in the innermost parsec of the Galactic nucleus. Comparing the spatial distribution and kinematics of the massive stars with observations of IRS 16, a group of young He I stars near the Galactic center, we argue that this association may have formed in this way.Comment: 15 pages, Accepted for publiction in Ap

Star Cluster Ecology: VII The evolution of young dense star clusters containing primordial binaries

We study the first 100Myr of the evolution of isolated star clusters initially containing 144179 stars, including 13107 (10%) primordial hard binaries. Our calculations include the effects of both stellar and binary evolution. Gravitational interactions among the stars are computed by direct N-body integration using high precision GRAPE-6 hardware. The evolution of the core radii and central concentrations of our simulated clusters are compared with the observed sample of young (about 100Myr) star clusters in the large Magellanic cloud. Even though our simulations start with a rich population of primordial binaries, core collapse during the early phase of the cluster evolution is not prevented. Throughout the simulations, the fraction of binaries remains roughly constant (about 10%). Due to the effects of mass segregation the mass function of intermediate-mass main-sequence stars becomes as flat as $\alpha=-1.8$ in the central part of the cluster (where the initial Salpeter mass function had $\alpha=-2.35$). About 6--12% of the neutron stars were retained in our simulations; the fraction of retained black holes is 40--70%. In each simulation about three neutron stars become members of close binaries with a main-sequence companion. Such a binary will eventually become an x-ray binary, when the main-sequence star starts to fill its Roche lobe. Black holes are found more frequently in binaries; in each simulated cluster we find about 11 potential x-ray binaries containing a black hole. Abstract abbreviated....Comment: MNRAS in pres

Short gamma-ray bursts from binary neutron star mergers in globular clusters

The first locations of short gamma-ray bursts (GRBs) in elliptical galaxies suggest they are produced by the mergers of double neutron star (DNS) binaries in old stellar populations. Globular clusters, where the extreme densities of very old stars in cluster cores create and exchange compact binaries efficiently, are a natural environment to produce merging NSs. They also allow some short GRBs to be offset from their host galaxies, as opposed to DNS systems formed from massive binary stars which appear to remain in galactic disks. Starting with a simple scaling from the first DNS observed in a galactic globular, which will produce a short GRB in ~300My, we present numerical simulations which show that ~10-30% of short GRBs may be produced in globular clusters vs. the much more numerous DNS mergers and short GRBs predicted for galactic disks. Reconciling the rates suggests the disk short GRBs are more beamed, perhaps by both the increased merger angular momentum from the DNS spin-orbit alignment (random for the DNS systems in globulars) and a larger magnetic field on the secondary NS.Comment: 13 pages, 2 figures, accepted for publication in Nature Physics (Feb. 2006

Contract Enforcement in Transition

The mix of formal and informal mechanisms for contract enforcement is examined using survey data from Russia, Ukraine, Romania, Poland, and Slovakia. Using the size of trade credit to quantify the success of contracting, we ask: Do the courts have a perceptible effect on contracting? When can a firm rely on its customer to repay trade credit voluntarily? Which is more effective, the courts or relational contracting? Do trade associations play a role in contract enforcement? Does relational contracting entail inefficiencies? Is the reliance on relation contracting merely a transitory phenomenon, reflecting the inadequacy of these countries' legal systems?

Property Rights, Finance, and Entrepreneurship

Is investment constrained more by insecure property rights or by limited external finance? For five transition economies in Eastern Europe and the former Soviet Union we find that weak property rights limit the reinvestment of profits in startup ma nufacturing firms. Access to credit does not appear to explain differences in investment. At least in the early stages of post-communist reform, retained earnings appear to have been enough to finance the investments that managers wanted to make.

Property Rights and Finance

Which is the tighter constraint on private sector investment: weak property rights or limited access to external finance? From a survey of new firms in post-communist countries, we find that weak property rights discourage firms from reinvesting their profits, even when bank loans are available. Where property rights are relatively strong, firms reinvest their profits; where they are relatively weak, entrepreneurs do not want to invest from retained earnings.